The present invention relates generally to electronic counter countermeasure techniques, and more specifically to an antijamming antenna for use in line of sight (LOS) links in a microwave communicator system.
The technical advances that have been made in the art of communications systems have coincided with the development of electronic countermeasures, known as ECM's, the purpose of which is to reduce or destroy the effectiveness of even the most effective communications system by jamming and disrupting the message traffic. The ECM's used to disrupt line of sight microwave communications links are interfering jammers which are located in the main beam and sidelove regions of the antenna pattern. The task of developing a suitable electronic counter countermeasure (ECCM) tailored for these microwave communications links is alleviated to some degree by a number or prior art techniques which allow both communications systems and radar tracking systems to filter out interference from repeater and spoofing jammers. However, the microwave communications links enjoy a distinct advantage over the tracking radar systems in that the exact direction of the desired incoming communication signal is known.
A jammer can be located in the main beam of a line-of-sight (LOS) receiving antenna. This jammer is likely to be displaced from the exact direction of the desired communication signal, either vertically or horizontally or both. An antenna system is desired that will permit an adaptive processor to cancel the jamming signal from such a main-beam jammer while retaining as much as possible of the desired communication signal.
A jammer can be located in a sidelobe of an LOS receiving antenna. An antijamming (AJ) antenna system is desired that will permit an adaptive processor to cancel the jamming signal from such a sidelobe jammer. If one jammer is in the main beam and another jammer is in a sidelobe, it is desired that both jamming signals be cancelled.
Where there is no jammer or the jammer is weak, it is possible for the adaptive system to attack and perhaps to cancel the desired communication signal. An objective for the AJ antenna system is to prevent this from happening. This objective should be accomplished by a spatial (antenna pattern) discriminant between the communication and jamming signals, because a spatial discriminant does not require techniques that utilize particular properties of the communication signal format or modifications to the signal waveform.
A jammer in the main beam creates a strong jamming signal because of the high gain of the narrow main beam. To cancel this jamming signal without introducing an excessive amount of receiver noise, an auxiliary jamming signal is needed that is also strong. Thus an auxiliary antenna pattern having high gain is desired. This could be obtained from a second antenna having a size comparable with the original one. However, it can also be obtained from the original reflector antenna by using a new feed. The antenna with a new feed provides not only the original main beam but also a new beam or beams that differ from the original one in some respect. The new beams are narrow high-gain beams, as desired for cancelling a main-beam jammer.
A jammer in a sidelobe of the antenna can be effectively cancelled by an auxiliary jamming signal that comes from an auxiliary antenna having a gain comparable with the sidelobe gain of the main antenna. This gain is much lower than the main-beam gain. Therefore, a rather small auxiliary antenna can be used. This permits a wide auxiliary pattern to be obtained, so that many sidelobes can be covered by any one auxiliary antenna. Therefore, only a few auxiliary antennas may be needed to cover all the sidelobes of the main antenna.
It should be mentioned that some of the sidelobes of the narrow auxiliary beam that is intended to cancel a main-beam jammer will also cover the main antenna sidelobes. This might permit the occasional use of the narrow auxiliary beam to cancel a sidelobe jammer. However, this would not be a reliable approach for sidelobe jammer cancellation because there would be substantial angular regions where the sidelobes would not be adequately covered. Furthermore, simultaneous jamming by a main-lobe jammer and a sidelobe jammer would be likely to defeat such an AJ system.
To summarize, a main-beam jammer is best handled by using a new feed in the reflector antenna that provides one or more narrow high-gain auxiliary beams in addition to the original main beam. A sidelobe jammer is best handled by the addition of relatively small auxiliary antennas that provide wide patterns.
A spatial discriminant is needed between the desired communication signal and a main-beam jammer signal. This discriminant is best achieved by an auxiliary pattern that is a monopulse difference pattern. Two such difference patterns are available simultaneously in a dual-plance monopulse antenna.